The present invention relates to the manufacture of metal-clad cermet articles, and more particularly to annular-shaped metal-clad cermet articles.
Cermets consist of ceramics embedded in metals. The conventional production method produces flat or near-flat plates.
Many applications have been developed for cermets. For example, Many types of armor are made from cermets. With the proper choice of particle sizes and materials, the cement can provide high resistance to assault and accidents. All of these excellent properties follow from the fundamental characteristic of a cermet: the ability to encapsulate variable quantities of different ceramic particles into a monolithic high-integrity metal matrix. The usual problems (low thermal conductivity, low ductility) that have prevented the using of ceramicist, for example in a spent nuclear fuel (SNF) cask, are avoided by the metal matrix. Cermets are also used in some extreme applications such as brake shoes, tool bits, and nuclear fuel assemblies in some reactors.
Many types of cermets have been manufactured using a wide variety of ceramics and metals. UO2-steel cermets have been manufactured and used as nuclear fuels in several research and test reactors, therefore there exists a massive experience base. The conventional process is the xe2x80x9cpicture framexe2x80x9d method (FIG. 1). A picture frame assembly is constructed with a bottom sheet 1 of metal and picture frame 2 of metal on top of the bottom sheet 1. For example, a mixture 3 of UO2 particles and steel particles is used to fill the space where the picture would be. A sheet of steel 4 is placed over the picture frame and the pieces of steel are welded together. The entire assembly is heated to an appropriate sintering temperature and the space within the particulate mixture is vacuum degassed. The heated and evacuated picture frame assembly 5 is sent through a rolling mill 6. The combination of temperature and pressure consolidates the particles yielding UO2 particles embedded in a continuous solid phase of steel. The steel particles are welded together by the rolling process to produce a metal-clad cermet plate 7.
Major difficulties have been experienced in fabricating cylindrical shells from cermets prepared by the picture frame method. Cermets are very difficult to weld and very difficult to form. This makes it very difficult to bend a flat sheet into a cylinder, weld the edge, and produce a cylindrical shell. For an application such as a SNF cask the cask walls may be 10 to 30 cm thick. This is far beyond the forming and welding technology that exists for cermet objects. Especially in thick sections, major problems are encountered in bending a flat cermet into a cylindrical form because the cermets have variable properties throughout the thickness of the material. To the present time, only partial sections of cylindrical shape (curved plates) have been manufactured, and those in thin sections.
It has long been desired to make thick-walled annular shapes such as cylinders with wall thicknesses in the range of 10 to 30 cm. Welding cermets of any thickness has always been questionable. Welding cermets of this thickness is beyond reality.
The present invention is a method to fabricate annular articles including, but not limited to, annular articles in the shape of bodies of revolution (such as cylinders, cones, etc.) and thus not requiring the rolling and welding of flat plates and the like to fabricate annular articles.
Accordingly, objects of the present invention include a metal-clad, cermet-filled, annular-shaped article which is formed by consolidating and sintering a mixture of metal powder and ceramic powder contained between an inner shell and an outer shell, thus avoiding problems associated with manufacturing flat plates of the clad cermet, and further rolling or otherwise forming and welding flat plates into annular shapes. Specifically, the construction of depleted uranium dioxide-steel cermet articles for use as casks for spent-nuclear fuel elements and the like is addressed.
Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a metal-clad, cermet-filled, annular-shaped article formed by the process steps of: providing a metallic, annular, hollow form having an inner wall and an outer wall; filling the hollow form with a particulate mixture of ceramic and metal; closing, evacuating, and hermetically sealing the form; heating the filled, evacuated, and sealed form to an appropriate sintering temperature; and applying force between the inner wall and the outer wall of the heated and evacuated, filled and sealed form to consolidate the particulate mixture into solid cermet to produce a metal-clad, cermet-filled, annular-shaped article.
In accordance with another aspect of the present invention, the foregoing and other objects are achieved by a method for making a metal-clad, cermet-filled, annular-shaped article comprises the steps of: providing a metallic, annular, hollow form having an inner wall and an outer wall; filling the hollow form with a particulate mixture of ceramic and metal; closing, evacuating, and hermetically sealing the form, heating the filled, evacuated, and sealed form to an appropriate sintering temperature; and applying force between the inner wall and the outer wall of the heated and evacuated, filled and sealed form to consolidate the particulate mixture into solid cermet to produce a metal-clad, cermet-filled, annular-shaped article.